The present invention relates to a process for preparing biphenols, and, more particularly, to a process whereby a substituted phenol is directly oxidized to the corresponding biphenol.
Biphenols have found wide utility as bactericides, chemical intermediates, copolymers and antioxidants. For example, biphenols, especially 2,2',6,6'-tetraalkyl-substituted 4,4'-biphenols are used to stabilize animal and vegetable fats and oils, gasoline, rubber compounds and the like. Moreover, 4,4'-biphenol, which can be prepared by dealkylation of substituted biphenol, especially 2,2',6,6'-tetratertiary butyl-4,4'-biphenol, is an excellent comonomer for use in preparation of various condensation polymers and copolymers.
In U.S. Pat. No. 4,238,627, a one-step process is disclosed wherein a phenol is contacted with up to a stoichiometric amount of oxygen in the presence of a catalytic amount of a base to prepare 4,4'-bis-(2,6-dihydrocarbylphenols) substantially free of quinones.
In U.S. Pat. No. 3,631,208, the process of reducing diphenoquinones by contacting with phenols is explained. The diphenoquinones were prepared by oxidative coupling of phenols in the presence of a catalyst, particularly an amine basic cupric salt complex.
In U.S. Pat. No. 3,562,338, a similar process is disclosed wherein a phenol is oxidatively coupled to form a diphenoquinone by contacting with oxygen in the presence of an alkali metal hydroxide catalyst and thereafter reducing the diphenoquinone by contacting with a phenol in the substantial absence of oxygen and recovering the biphenol product.
Previous processes have first prepared a diphenoquinone by oxidatively coupling a phenol. Disadvantageously the prior art processes have been unduly inefficient. In particular, these processes have been characterized by the evolution of large amounts of heat due to the highly exothermic nature of the process. This fact necessitates the use of expensive thermal transfer equipment and procedures in order to maintain the reaction under control. The problem is compounded by the necessity to employ elevated temperatures in order for the oxidative coupling process to proceed at an acceptable rate. Prior art processes have further required inordinately long reaction times due to the kinetic limitations of the coupling process and have been limited by concommitant formation of undesired isomers thereby lowering selectivities to the desired p,p'-biphenol product.
Finally, and most significantly, prior art processes have had associated therewith a significant risk of explosion in view of the combination of low molecular weight phenols of relatively high volatility with oxygen at elevated temperatures.
It would be desirable to provide an improved process for preparing biphenols operating at greater efficiencies. It would also be desirable to provide an improved process for preparing biphenols that provides a reduced risk of explosion. It would further be desirable to provide an improved process for preparing biphenols that operates at improved selectivities and in shortened reaction times.